1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a printer, and a facsimile, or a multifunction product of these devices using an electrophotographic system, and to a developing device and a process cartridge provided therein. More particularly, the present invention relates to a developing device in which at least two conveying members among a plurality of conveying members that forms a circulation route through which a developer is conveyed along the longitudinal direction are arranged so as to face a developer carrier and in which a developer regulating member for regulating an amount of developer at an upstream side of a developing region is disposed below the developer carrier, and to a process cartridge and an image forming apparatus.
2. Description of the Related Art
Conventionally, there has been known a technology for a developing device, which stores a two-component developer containing toner and carrier (which also includes a case of adding external additives or the like) in an image forming apparatus such as a copier and a printer, and in which at least two conveying members among a plurality of conveying members for forming a circulation route through which a developer is conveyed along the longitudinal direction are vertically arranged and in which a developer regulating member for regulating an amount of developer at an upstream side of a developing region is disposed below a developer carrier (e.g., see Japanese Patent Application Laid-open No. H11-174810, Japanese Patent Application Laid-open No. 2008-26408 and Japanese Patent No. 3950735).
The developing device using the two-component developer has a toner supply port provided at a part of the developing device, and toner is appropriately supplied into the developing device through the toner supply port depending on toner consumption in the developing device. The supplied toner is stirred and mixed with the developer in the developing device by a conveying member (a stirring and conveying member) such as a conveying screw. Part of the stirred and mixed developer is supplied to a developing roller (developer carrier). The developer carried on the developing roller is controlled to an appropriate amount by a doctor blade (developer regulating member) disposed below the developing roller, and then, the toner in the two-component developer is caused to adhere to a latent image formed on a photosensitive drum at an opposite position to the photosensitive drum (image carrier). A magnet is fixed inside the developing roller, and a plurality of magnetic poles is formed around the developing roller by the magnet.
A first conveying member (a supplying screw) and a second conveying member (a collecting screw) are arranged vertically in the developing device disclosed in Japanese Patent Application Laid-open No. H11-174810 and the like. These two conveying members form a circulation route of the developer. The first conveying member arranged in the lower side of the developing device supplies the developer to the developing roller at a position of a developer-scoop-up magnetic pole while conveying the developer along the longitudinal direction. The second conveying member arranged in the upper side of the developing device conveys the developer released from the developing roller at a position of a developer-releasing magnetic pole along the longitudinal direction (a direction opposite to the conveying direction of the first conveying member). The downstream side of a conveying path formed by the first conveying member (a first conveying path) communicates with the upstream side of a conveying path formed by the second conveying member (a second conveying path) through a first relay portion. The developer having reached the downstream side in the first conveying path stays at the position and is pushed up to reach the upstream side in the second conveying path. Here, the toner supply port is provided at an upstream side in the second conveying path and new toner is appropriately supplied through the toner supply port. The upstream side in the first conveying path communicates with the downstream side in the second conveying path through a second relay portion. The developer having reached the downstream side in the second conveying path drops in the second relay portion by its own weight, and is moved to the upstream side in the first conveying path.
The developing device in which the conveying members are vertically arranged in the above manner can be made small in the horizontal direction as compared with a developing device in which a plurality of conveying members is horizontally arranged (see, for example, FIG. 19 in Japanese Patent Application Laid-open No. 2008-26408). Therefore, the compact developing device is often used in tandem type color image forming apparatuses in which a plurality of developing devices is horizontally arranged. A developing device, in which the conveying members are vertically arranged and a supplying route (the first conveying path) of a developer to the developer carrier is separated from a collecting route (the second conveying path) of a developer released from the developer carrier, can reduce a concentration deviation in a toner image formed on the image carrier because the developer after a developing process is hard to be contained in a developer to be carried on the developing roller and be supplied to the developing process, as compared with the developing device in which the conveying members are horizontally arranged (see, for example, FIG. 19 in Japanese Patent Application Laid-open No. 2008-26408).
As explained above, an image forming apparatus that includes a developing device with the doctor blade (developer regulating member) disposed below the developing roller can reduce the length of a paper conveying path from a paper feeding unit (paper storage) disposed in the lower part of the image forming apparatus to a paper ejection tray, as compared with an image forming apparatus that includes a developing device (see, for example, FIG. 19 in Japanese Patent Application Laid-open No. 2008-26408) with the doctor blade disposed above the developing roller. Thus, first print time in the tandem type color image forming apparatuses can be reduced (see, for example, FIG. 1 in Japanese Patent Application Laid-open No. 2008-26408). Furthermore, because a layout that the paper ejection tray is disposed in an upper side of the image forming apparatus can be easily provided even if the paper conveying path is made comparatively shorter, this layout is often used in the tandem type color image forming apparatuses which are made small in the horizontal direction.
In the developing devices disclosed in Japanese Patent Application Laid-open No. H11-174810, Japanese Patent Application Laid-open No. 2008-26408 and Japanese Patent No. 3950735, the developer released from the developing roller after the developing process at the position of the developer-releasing magnetic pole may be again carried on the developing roller in the second conveying path. Such an inconvenience becomes a non-negligible problem especially in the developing device in which the second conveying path (collecting route) is disposed above the first conveying path (supplying route). Moreover, the inconvenience becomes more significant when the fluidity of the developer decreases caused by degradation of the developer over time.
Once the inconvenience occurs, the developer after the developing process (developer in which the toner is consumed) is contained in a developer which is to be carried on the developing roller and be supplied to the developing process, and this causes concentration deviation to occur in a toner image formed on the image carrier.
In order to suppress the inconvenience, there are possible measures that provide a separation member for separating the first conveying path from the second conveying path so as to face the developer carrier and that optimize the position of the developer-releasing magnetic pole and the position of the second conveying path. Furthermore, in order to reliably release the developer from the developing roller at the position of the developer-releasing magnetic pole, there are also possible measures that set magnetic force of two magnetic poles having the same polarity and forming the developer-releasing magnetic pole to be sufficiently large.
In these cases, however, there is a possibility to cause such an inconvenience that the developer released from the developing roller at the position of the developer-releasing magnetic pole is again carried (re-carried) on the developing roller by the magnetic force of an upstream-side magnetic pole of the two magnetic poles that form the developer-releasing magnetic pole. Once the inconvenience occurs, the load of the developing roller at the position where the developer is re-carried thereon becomes heavy, which causes rotational torque of the developing roller to increase.